Depending if the site has a direct competitor and non-sticky customers, you can often get accurate loss estimates from outages. For example, friends of mine at Doordash would know when UberEats was down by the corresponding spike in traffic to their app. The competitor captures all the lost traffic.

Most enterprises will have a harder time quantifying losses, as some percentage of customers will come back later. To understand that, you need to look for a drop in completed purchase rates compared to site visits.

For a SaaS, it's even more difficult, as customers are often held captive by long contracts and might tolerate SLA breaches up to a certain point. A reasonable, though fictional, proxy would be the revenue for the contract pro-rated against the uptime during that period.